Active millimeter wave imaging is useful in security screening for concealed threats because of the property of millimeter waves to penetrate clothing. The technique is employed in personnel screening systems to detect concealed explosives, narcotics, weapons and other contraband in or beneath an individual's clothing because of the noninvasive nature of the scanning method and lack of physical contact required during an inspection. The detection method relies on the identification of anomalies in the images produced by the reflected radiation returned from the body of an inspected subject.
Millimeter wavelengths are relatively long and inherently lower in resolution than images formed from radiating means using shorter wavelengths. Because the detection of objects using millimeter wavelengths relies on the contrast between the millimeter wave reflection intensity from a body and the millimeter wave reflection intensity from an adjoining object or anomaly; the system cannot distinguish between benign objects and contraband. Such contrast based systems are not geared toward making composition based determinations other than metallic vs. opaque (with shape, or structural based determination methods also known in the prior art). Metallic objects will have a high reflection intensity and low emissivity, producing a more significant contrast with the reflection intensity and emissivity of the person, than an opaque non-metallic material. The inability to determine composition reduces detection to anomaly based detection instead of identity based detection, thus the detection of anomalies such as keys or other benign objects lead to a significant rate of false alarms which is problematic in implementing these systems as a personnel screening tool. The invention described herein relates to a method for identifying the composition of an anomaly detected using active millimeter wave reflectometry, so that the contraband may be distinguished from benign material through its relative permittivity or other dielectric properties.